Most devices for protecting thermocouple elements of immersion pyrometers are made either of a metal, such as cast iron, stainless steel or molybdenum, which may be uncoated or coated with a ceramic layer (e.g. as disclosed in U.S. Pat. No. 4,721,533 issued Jan. 26, 1988 to Phillippi et. al.), or they are made of a fine-grained ceramic material, such as sintered alumina, magnesia or zirconia, or sintered mixtures of such materials with graphite (e.g. as disclosed in U.S. Pat. No. 5,209,571 issued May 11, 1993 to Kendall).
Protective devices made from non-refractory metals, such as cast iron, have low resistance to corrosion by molten metals and low resistance to oxidation. These low resistance levels can be increased by the provision of ceramic coatings, but such coatings are generally thin (no more than a few millimeters) and may peel away from the protective device during service. Protective devices made of refractory metals such as molybdenum have improved resistance to corrosion but these metals are much more expensive than non-refractory metals and still have low resistance to oxidation.
Fine-grained ceramic materials, such as sintered alumina and magnesia, are resistant to oxidation and may be resistant to corrosion by various molten metals. These characteristics, added to their low porosities, make these materials suitable for the protection of thermocouple elements against corrosion by molten metals in the presence of corrosive gases. However, such materials generally have a low mechanical strength and a poor resistance to thermal shocks. These disadvantages could, for some applications, be overcome by the use of sialons as the ceramic materials. However these fine-grained ceramics are expensive and their use is limited mainly to the non-ferrous industry since they have a poor resistance to corrosion by molten iron and steel.
Finally, none of the known protective devices permits a commercially feasible continuous temperature reading of molten iron or steel by means of an immersion pyrometer. In the ferrous industry, immersion pyrometers having a thermocouple element supported inside a cardboard sheath are mostly used at present. However, such thermocouple devices permit only one temperature measurement and must be replaced after each use. It will be appreciated that this is inconvenient and ultimately expensive.